Method of initiating combustion in an oil reservoir



United States Patent a METHOD OF INITIATIN G CONIBUSTION IN AN OIL RESERVOIR William A. Bruce, Tulsa, Okla., assignor to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Application September 8, 1954, Serial No. 454,859

3 Claims. (Cl. 166-39) This invention concerns a technique for improving the recovery of petroleum oil from subterranean reservoirs by a combustion process. The invention is particularly concerned with a procedure for initiating combustion in a petroleum reservoir.

In recovering crude petroleum oil from oil reservoirs in the earth the first phase of production activity is ordinarily called primary production. During this period oil is chiefly produced from the bore holes penetrating oil bearing formations as a result of natural pressure exerted on the oil formation. This pressure may be gas pressure, natural water pressure, or the like. Primary recovery methods cannot always be relied upon to produce more than a minor portion of the oil actually in the reservoir. Thus, the natural pressure referred to, relied upon to cause primary production, will be depleted while substantial amounts of oil remain in the reservoir.

One of the recently suggested techniques relating to secondary recovery is to actually burn at least a portion of oil in place in the reservoir. Thus, by supplying a combustion Supporting gas, such as air, to the oil bearing formation, actual combustion of the oil in the reservoir can be accomplished. The combustion serves to heat the reservoir or oil in the reservoir in a manner reducing the viscosity of the oil and improving its flow through the formation to production wells. Again, at least some degree of cracking or visbreaking commonly occurs, serving to reduce the gravity and viscosity of the oil so as to improve its ultimate recovery. Finally the hot combustion gases serve as a drive medium to force oil through the reservoir.

The present invention is concerned with this general technique of underground combustion conducted in an oil reservoir. The invention is specifically concerned with a technique for effectively initiating the desired combustion in the oil formation.

It has been supposed that initiating of underground combustion could be accomplished without particular difi'iculty. Thus, it has been suggested that combustion can be started simply by bringing air together with a combustible gas into an injection well and igniting this mixture by means of an electrical ignition device or other means. It has also been suggested that solid combustible material such as charcoal, for example, could be placed in an injection well adjacent the oil bearing formation so that by burning the charcoal combustion of oil in the formation itself could be initiated. However, it has been found that real problems are encountered in satisfactorily initiating combustion in a reservoir and techniques such as those referred to have not been entirely satisfactory.

The problem to which this invention relates can be appreciated by reference to a reported field test of underground combustion. In the particular case referred to, the production formation had a thickness of about feet and it was attempted to initiate combustion in the oil sand for this thickness about an injection well. However, by using the ignition techniques referred to, it was found that only the top 8 feet of the 20-foot thick forma- 2,796,132 Patented June 18, 1957 tion was satisfactorily burned out. There was evidence to show that combustion of the lower portion of the formation was obstructed by plugging due to accumulation of coke or other solid residue. From this it would appear that oil from the formation itself was converted to coke or heavy plugging residues when combustion was initiated, thus essentially plugging off a substantial portion of the producing formation in the injection well. It would also seem probable that such plugging resulted in seriously decreasing the permeability of the formation about the injection well so as to complicate continued injection of air for continuation of combustion through the reservoir.

The nature of this problem can also be understood by reference to laboratory experiments which were conducted to simulate low temperature oxidation of crude oil. In these tests a sandstone core was employed having a length of one foot and a diameter of 2 inches. The core was first completely filled with oil, and oxygen was then forced through the core until breakthrough; that is, until at least some of the oxygen was forced completely through the core. The core was then maintained in an atmosphere of oxygen at a temperature of 300 F. and a pressure of 300 lbs. per square inch, approximately, for a period of 72 hours. At the end of that period, the core was examined in order to determine the effect of the oxidation treatment on the crude oil in the core. It was found that the core was severely coked or plugged by a heavy residue which was almost solid in nature. The residue had the general appearance of an air-blown asphalt and resulted in cutting down the permeability of the core very greatly. It may be seen from this data that initiation of underground combustion in a manner preventing plugging of the injection well is a problem of some significance.

It is thus the principal purpose of this invention to provide a technique for starting underground combustion from an injection well in a producing formation. The invention serves to initiate combustion while preventing the serious decrease in the injection capacity of the injection Well which would otherwise be encountered. Again it is the purpose of this invention to secure substantially even combustion over the entire depth of a producing formation while preventing isolation of combustion to a thin portion of the formation. By initiating combustion as disclosed hereinafter, it is possible to substantially improve the ultimate oil recovery during the history of an underground combustion recovery process.

In the practice of this invention, in order to initiate combustion from an injection well, a low viscosity liquid hydrocarbon material is first introduced into the formation so as to drive oil back from the vicinity of the injection well. The liquid hydrocarbon must be a material that is mutually soluble with crude oil so that effective displacement of the oil occurs by the so-called miscible fluid displacement mechanism. Thus, the fluid serves to attain a more or less piston-like displacement of the crude oil with substantially complete elimination of crude oil from the vicinity of the injection well. Solvents which can preferably be employed for this purpose are low molecular weight normally gaseous, liquefiable hydrocarbons and particularly those hydrocarbons having from 2 to 5 carbon atoms. Of these, propane and butane are particularly attractive for use and the mixture known as liquefied petroleum gas can attractively be employed.

For the purposes of this invention, it is only necessary to inject suificient solvent so that crude oil will be forced a substantial distance from the vicinity of the injection well before combustion is initiated. In general, this technique may be used to clean out the formation for a radius of about 50 to feet about the injection well. A relatively minor amount of solvent is required to 3 achieve this. In general, about to 50 percent hydrocarbon pore volume of the portion of the reservoir to be cleaned of crude oil is required.

In order to further minimize the amount of solvent required, it is best to inject the selected amount of solvent into the injection well as a single slug or bank. Thereafter gas or water can be injected into the injection well so as to force the slug of solvent through the reservoir adjacent the injection well to secure eifective cleanup of the reservoir in the immediate vicinity of the injection well. It is particularly preferred, however, to employ natural gas as the drive fluid, since subsequent combustion is simplified when a combustible fluid such as natural gas is employed for this purpose.

The mechanism of this invention can be understood by reference to simulated drive experiments; conducted under carefully controlled conditions.- For. these tests, a core was prepared, constructed of sand and; plastic so as to have a permeability of 10 md. Thecore was 20 feet long and 2 inches in diameter and was sealed in a jacket in a horizontal position. Taps penetrating the jacket and, leading to the core were. positioned, at each 4 feet along the core. In the experiments. which were conducted, the core was fully saturated with decane used' to simulate oil in place in a reservoir. One thousand nine hundred ninety ccs. of decane were required for this purpose. Thereafter, 0.13 hydrocarbon pore volume of propane was injected into one end of the core and this propane was driven through the core by a gas drive until all the propane was recovered from. the opposite endv of the core. At the end of this treatment the oil remaining in each of the 4-foot sections of the core was determined. It was found that only 2.5 ccs. of oil remained in the first 4-foot section of the core at the end of the core where the propane was injected. The successive 4-foot lengths of core were. foundto have respectively 4.5, '25, 41 and 63 ccs. of oil remaining therein. It will be noted from these data that substantially complete removal of oil can be achieved from the portion of a reservoir about an injection well by employing minor amounts of propane as an oil solvent.

In accordance with this invention, therefore, in order to initiate combustionin anoil reservoir, a slug of a light hydrocarbon such as propane is first injected into the reservoir through an injection well. This slug or bank of solvent is then forced back into the reservoir by means of a fluid drive. As stated, natural gasis preferably employed to drive the solvent into the reservoir. The amount of drive fluid to be injected is not critical although approximately 2 to 20 times (measured in volumes of gas or liquid under operating; pressures and temperatures), the amount of solvent should be employed for this purpose. Thereafter a combustible gas mixture which may constitute natural gas and oxygen is injected into the injection well and combustion is initiated by means of a high voltage spark or other electrical igniter. Combustion will then move through the reservoir in the producing formation as the flame front spreads from the injection well. Because of the substantially complete elimination of all crude oil from the vicinity of the injection well, the problems of partial or complete plugging are substantially avoided, and ultimate oil recovery is substantially increased.

What is claimed is:

1.. Inthe recovery ofoil from an underground reservoir by a technique involving the combustionof a portion; of the oil in place with a combustion supporting gas, the

'improved method of initiating: combustion of the oil which comprises, in combination, the steps of injecting a bank of a light hydrocarbon liquid solvent for crude oil through an injection well into the reservoir to sweep reservoir oil from the immediate vicinity of the injection well, thereafter injecting a fluid through the injection well to force the bankof solvent further into the reservoir,

injecting a combustible gas into the reservoir through the injection well, initiating combustion of the combustiblegas in the solvent swept region of the reservoir, and

advancing the resulting flame front through the reservoir to effect "combustion of said oil in. place.

2. Amethod as defined in claim l in which the bank of solvent constitutes about 1-0 percent to percent.

hydrocarbon pore volume based. on the volume of the reservoir about the injection well for a distance of about 50 to feet. 3;. In thereeovery of the'oil in place with a combustion supporting gas, the improved. method of initiating combustion of the oil which comprises, in combination, the; stepsof injectinga bank of a normally gaseous, liquefied petroleum gas through an injection; well into the; reservoir: to sweep oil. from the portion ofthe reservoir in theimmediate vicinity of the; injection well, thereafter injecting; natural, gas through, the injection well to force the, bank of liquefied petroleum gas further into the reservoir, injecting; a combustion-supporting gas; through the injection well and. into the reservoir, initiating combustion. of the natural gas with the combustion-supporting gasin the portion of the reservoir from which the oil, had been swept,v and advancing the combustion through the reservoir to effect combustion of the oil in place.

References Cited in the file of thispatent UNITED STATES PATENTS 7 Heilman Sept. 20, 1955 oil from an underground reservo r by a technique involving the combustion of a; portion of 

1. IN THE RECOVERY OF OIL FROM AN UNDERGROUND RESERVOIR BY A TECHNIQUE INVOLVING THE COMBUSTION OF A PORTION OF THE OIL IN PLACE WITH A COMBUSTION SUPPORTING GAS, THE IMPROVED METHOD OF INITIATING COMBUSTION OF THE OIL WHICH COMPRISES, IN COMBINATION, THE STEPS OF INJECTING A BANK OF A LIGHT HYDROCARBON LIQUID SOLVENT FOR CRUDE OIL THROUGH AN INJECTION WELL INTO THE RESERVOIR TO SWEEP RESERVOIR OIL FROM THE IMMEDIATE VICINITY OF THE INJECTION WELL, THEREAFTER INJECTING A FLUID THROUGH THE INJECTION WELL TO FORCE THE BANK OF SOLVENT FURTHER INTO THE RESERVOIR INJECTING A COMBUSTIBLE GAS INTO THE RESERVOIR THROUGH THE INJECTION WELL, INITIATING COMBUSTION OF THE COMBUSTIBLE GAS IN THE SOLVENT SWEPT REGION OF THE RESERVOIR, AND ADVANCING THE RESULTING FLAME FRONT THROUGH THE RESERVOIR TO EFFECT COMBUSTION OF SAID OIL INPLACE. 